1. Field of the Invention
This invention lies in the field of explosive charges used in seismic geophysical prospecting.
More particularly, this concerns an improvement in the construction of long linear explosive charges in that they are made of a plurality of separate sticks which are attached by hinges between their abutting ends, so that after the string of sticks are placed in co-linear position, they can be wound with a helix of elongated explosive material in such a way that the hinged joints can be rotated and the charge folded into a plurality of parallel sticks without disturbing the explosive material.
2. Description of the Prior Art
It is well known in the art that a linear explosive charge comprising a helix of linear explosive material when designed with a proper diameter and spacing between helical turns, provides an improved seismic record over the conventional type of concentrated point charges. explosive material, is necessary because the available explosives detonate at a velocity much greater than the velocity of propagation of seismic waves in the earth. The pitch of the helix is designed to provide a match with these velocities.
In spite of the noticeably improved seismic records recorded when these linear charges are used, the Geophysical Industry has been reluctant to utilize them for a number of reasons. One reason results from the experience that when such charges are loaded into a hole at the time the hole is drilled, and not shot for some considerable number of days, there are occasionally misfires, in the sense that the charge does not completely detonate. Such failures to completely detonate are expensive and time-consuming difficulties.
Part of the reason for the failure to detonate stems from two problems. One is that the paper tubes on which the explosives are wound sometimes becomes soft and slump in the water filling the hole and, consequently, the spacing between turns may become smaller, to the point where a cross-detonation occurs from one turn to the other which vitiates the usefulness of the charge.
A second and more important problem is that in the prior art the separate sticks of linear explosive charge were made with fastening means, such as screw threads, by which they can be attached end-to-end and inserted into the drill hole. However, when this is done, particularly where the two sticks must be rotated with respect to each other, there is no satisfactory way of joining the two ends of the linear explosive compound, such as Primacord, for example, so that there will be positive detonation across the joint between the Primacord on each of the adjacent sticks.
The most common construction of the explosive sticks involves sealing the two ends of the Primacord and inserting the two ends, one from one stick, and the other from the other stick, through an opening between the sticks along the axis of the charge, so that the two lengths of Primacord are in intimate parallel contact. Under good dry conditions, at this kind of a joint, the detonation of one cord will cause the detonation of the other cord. However, for various reasons, such as leakage of water into the metal caps, or displacement of the two lengths of cord one from the other, or other reasons, misfires do occur at the junction between the two ends and, of course, the charge fails to detonate completely throughout the whole length.
In the prior art there has been some work done on utilizing a flexible tubular member such as a heavy walled rubber hose which is stretched linearly and rotated while Primacord is wound in a continuous length from one end to the other of the hose. This type of construction will definitely show fewer misfires than the type previously described in which separate sticks are coupled together. However, the time of the workmen in the field which is required to wind these separate charges is prohibitive except for special research assigns, and it cannot economically be justified in the routine operation of a seismic geophysical party. Furthermore, the average person working as a laborer on the seismic party is not well enough trained to "manufacture the distributed charge in place".